This invention relates to semiconductor optical devices with quantum well structures. More particularly this invention relates to the monolithic integration of transparent optical mode transformers with an electroabsorption modulator.
A typical electroabsorption (EA) modulator is composed of a semiconductor device, which has light coupled into and out of it by two optical fibers. The optimum optical beam profile for efficient modulation is not the same as the optimum optical beam profile for efficient fiber coupling. This is especially true in high speed EA modulators.
If efficient optical coupling into and out of the EA modulator is not achieved, then system performance is degraded owing to excessive optical losses. Likewise, if efficient modulation is not achieved within the EA modulator, then system performance may be degraded owing to poor signal quality. For optimum modulator performance, it is desirable to independently optimize the optical beam profile in the modulation region of the semiconductor device and at the fiber input and output coupling surfaces of the device.
One possible solution is the inclusion of mode expansion/contraction regions, which couple the optical signal into and out of the optical fibers with one optical beam profile, or mode, and couple the optical signal into and out of the modulation region of the semiconductor device with another beam mode.
There have been numerous attempts to independently optimize these sections. One technique, described by Johnson, et. al. (U.S. Pat. No. 6,162,655), uses a beam expansion technique, wherein the transfer of the optical mode from the modulation region to an underlying passive waveguide is through a bumped mode transfer section. The modulation region uses quantum wells optimized for modulation properties of a preselected beam. The underlying waveguide is optimized for beam expansion properties to allow optimum optical modes for both external fiber coupling and modulation.
Some loss at the input and output couplings may be unavoidable, but any optical loss within an EA modulator is highly undesirable. To avoid high optical transition loss between the waveguide and the modulation region, the thicknesses of all the layers in the transition region are desirably carefully controlled. This technique requires a large number of precise fabrication steps.
Another technique for independently optimizing the modulator region from the beam expander region was suggested by Ido, et. al. (U.S. Pat. No. 5,742,423). The application of a xe2x80x9cbutt-jointxe2x80x9d technique is used to achieve independently optimized regions on the modulator. In this technique, the modulation region is defined through etching and the mode expander is selectively grown. The mode transfers directly through the butt joint region between the modulation and mode expander regions. This technique has the advantage of the mode not being transferred vertically within the structure. The optical losses can be kept reasonably low, except for the potential of an abrupt interface with slightly different modal indices at the butt joint. This may cause a reflective loss if the interface is not truly adiabatic. This technique uses regrowth of epitaxial material on an etched structure. Epitaxial growth on etched surfaces can reduce yield due to possible non-uniform growth problems. Also, it can prove difficult to obtain proper mode matching between regions, which may lead to undesirable reflections or scattering.
Arakawa, et. al. (U.S. Pat. No. 5,757,833) disclose a selective area growth method to produce quantum well lasers. An integrated infrared laser and output waveguide, fabricated by this method is disclosed. The output waveguide is both transparent and, through selective area growth, is shaped so as to increase the optical mode size for better mode coupling of the laser output to an optical fiber. Selective area growth techniques limit the absolute amount of enhancement which can be achieved and the degree of transparency attainable in the mode expansion section, while retaining the quality and reliability of the device.
Lasers, such as those disclosed by Arakawa et al., must be concerned with saturable absorber effects, which may lead to non-linearity in the optical output power. For this and other reasons this technique has not widely used in laser devices. The technique of selective area growth of quantum wells is however widely deployed to monolithically integrate lasers with modulators where only a slight enhancement is necessary and the quality can be retained.
In addition, lasers require reflective elements for their operation. Arakawa et al. disclose using the cleaved surfaces of the selective growth areas as reflectors.
One embodiment of the present innovation is a monolithic single pass expanded beam mode active optical device for light of a predetermined wavelength and a predetermined beam mode. An exemplary a monolithic single pass expanded beam mode active optical device includes: a substrate; a waveguide layer coupled to the top surface of the substrate; a semiconductor layer coupled to the waveguide layer; first and second electrodes for receiving an electric signal coupled to the substrate and the semiconductor layer, respectively.
The waveguide layer includes a plurality of sublayers, forming a quantum well structure, which is responsive to the electric signal. The waveguide layer has three sections, two expansion/contraction sections and an active section, which extends between and adjacent to the two expansion/contraction sections. At least one of the plurality of sublayers varies in thickness within the expansion/contraction portions of the quantum well structure. The active portion of the quantum well structure interacts with light of the predetermined wavelength, responsive to the electric signal. Possible interactions of the active region with the light include: absorption in the case of an EA modulator or optical gain in the case of an SOA.
A further embodiment of the present innovation is a monolithic expanded beam mode EA modulator for modulating light of a predetermined wavelength, responsive to an electric signal. An exemplary monolithic expanded beam mode EA modulator includes: a substrate; a waveguide layer coupled to the substrate; a semiconductor layer coupled to the waveguide layer; and first and second electrodes for receiving the electric signal coupled to the substrate and semiconductor layer, respectively.
The waveguide layer includes a plurality of sublayers, which form a quantum well structure. This quantum well structure includes two expansion/contraction sections and an electroabsorption section. The thickness of at least one of the plurality of sublayers varies within the expansion/contraction sections. Also the expansion/contraction sections have a cutoff wavelength which is shorter than the predetermined wavelength. The electroabsorption section extends between, and adjacent to the two expansion/contraction sections. The cutoff wavelength of electroabsorption section has a first value, which is shorter than the predetermined wavelength, responsive to the on-voltage of the electric signal, and has a second value, which is longer than the predetermined wavelength, responsive to the off-voltage of the electric signal.
Another embodiment of the present invention is method of manufacturing a monolithic expanded beam mode electroabsorption modulator of the first embodiment. The first step of this method is to form the waveguide layer on a portion of the top surface of the substrate by selective area growth. The waveguide layer having: a waveguide index of refraction; an electroabsorption thickness in an electroabsorption portion of the waveguide layer that is greater than the thicknesses in remaining portions of the waveguide layer along the longitudinal axis; and a plurality of sublayers forming a quantum well structure, each of the sublayers including a waveguide material. Next, the semiconductor layer, having a semiconductor layer index of refraction, is formed on the waveguide layer. Then, the waveguide layer and the semiconductor layer are defined and etched to form, along the longitudinal axis: the electroabsorption section and the two expansion/contraction sections disposed on opposite sides of the electroabsorption section. The semiconductor layer is then planarized and first and second electrical contacts are formed on the substrate and the semiconductor layer, respectively.
Another embodiment of the present invention is an optical signal modulation system. An exemplary system contains: a laser to produce a light beam with a predetermined wavelength and oscillating in a first beam mode; an exemplary monolithic expanded beam mode EA modulator; and an optical fiber optically coupled to the monolithic expanded beam mode EA modulator and substantially optimized for low input loss and transmission of light beams oscillating in the first beam mode.
Yet another embodiment of the present invention is an extended range optical communication system. In an exemplary extended range optical communication system, a laser produces a light beam with a predetermined wavelength and a first beam mode. This light beam is optically coupled at the input end and transmitted along a first optical waveguide. The output end is optically coupled to a monolithic expanded beam mode optical amplifier. An exemplary monolithic expanded beam mode optical amplifier includes: an input surface substantially optimized for low input loss of light beams with the first beam mode; an expansion section to expand the beam mode of the light beam for increased confinement of the light beam; an optical amplification section, which includes a semiconductor gain medium for amplifying light of the predetermined wavelength; a contraction section to contract the beam mode of the light beam to about the first beam mode; and an output surface. The amplified light beam is optically coupled a second optical waveguide, which is substantially optimized for low input loss and transmission of light beams with the first beam mode.
Another exemplary embodiment of the present invention is a low-loss demultiplexer for demultiplexing a plurality of temporally offset channels, each of which is modulated at a channel bit rate and temporally offset from the remaining channels by less than a minimum time between bits. The input optical signal source is coupled into a monolithic expanded beam mode EA modulator which may be periodically modulated at the channel bit rate with the temporal offset of one channel of the input signal to select that channel. The resulting single channel signal is then optically coupled to a receiver.
Yet another exemplary embodiment of the present invention is an exemplary low-loss demultiplexer for demultiplexing a time division multiplexed (TDM) optical signal have a plurality of channels, each channel transmitted as blocks which are temporally interleaved with blocks of other channels. The exemplary low-loss demultiplexer includes: an optical beam splitter for splitting the TDM signal; a monolithic expanded beam mode EA modulator to select blocks of a single channel; and a buffer optically coupled to the output surface of the monolithic expanded beam mode electroabsorption modulator to store the selected blocks.